As filter media for filters, various ones have hitherto been proposed. For example, there have been proposed an airlaid multilayer filter medium having a fiber fineness gradient (for example, see Patent Document 1), one in which ultrafine fibers obtained by an electrospinning process are laminated on a surface layer of a general-purpose nonwoven fabric (for example, see Patent Document 2 and Patent Document 3), and the like.
Of these, the airlaid multilayer filter medium having a fiber fineness gradient make it possible to achieve low pressure loss and a high filter lifetime, but is insufficient for collecting extremely fine dusts. Further, the filter medium in which the ultrafine fibers are laminated on the surface layer of the general-purpose nonwoven fabric comes into a state where the surface layer is planarly coated with the ultrafine fibers, so that there has been a problem of an easy increase in pressure loss, easy omission of the fibers because of their insufficient adhesiveness with the nonwoven fabric as a substrate, or the like.
A nonwoven fabric composed of short-cut nanofibers has also been proposed. However, although this nonwoven fabric has been designed to place emphasis on achievement of initial efficiency, the fabric has not been considered to be sufficient yet in terms of the filter lifetime (for example, see Patent Document 4).